THURSDAY, April 11, 2019 (HealthDay News) -- Almost one in five multiple sclerosis patients may be misdiagnosed with the autoimmune disease, according to a new study.

Of 241 previously diagnosed multiple sclerosis (MS) patients referred to two major Los Angeles medical centers for treatment, nearly 18% did not actually have the autoimmune disease, the researchers found.

Those patients spent an average of nearly four years being treated for MS before receiving a correct diagnosis, the study authors said.

"I've seen patients suffering side effects from the medication they were taking for a disease they didn't have," said study leader Dr. Marwa Kaisey, from Cedars-Sinai Medical Center in Los Angeles.

"Meanwhile, they weren't getting treatment for what they did have. The cost to the patient is huge -- medically, psychologically, financially," Kaisey said in a Cedars-Sinai news release.

The most common correct diagnosis among the patients misdiagnosed with MS was migraine (16%), followed by radiologically isolated syndrome, a condition in which patients do not experience symptoms of MS even though their imaging tests look similar to those of MS patients.

Other diagnoses included nerve damage and a disorder of the vertebrae called spondylopathy, the findings showed.